The present invention relates to the manufacture of ceramic matrix composite articles, and more particularly to a method for making a fiber-reinforced ceramic matrix composite panel comprising one or more integral stiffeners which exhibits high rigidity as well as excellent strength.
Several methods for making ceramic matrix composite structures of complex shape are known. U.S. Pat. No. 4,776,866, for example, teaches the fabrication of complex shapes such as honeycomb bodies wherein ceramic batches comprising powdered ceramic material in combination with inorganic whiskers or chopped fibers are extruded as green honeycomb bodies and thereafter fired to consolidate them into dense fiber reinforced ceramics.
U.S. Pat. No. 4,921,528 discloses another fabrication method wherein a fiber-containing feedstock produced by chopping fiber-reinforced glass rods or prepregs is reheated to the softening point of the glass, and thereafter reshaped at high temperatures into the desired product configuration. Depending on the glass viscosity at the shaping temperature, molding, stamping, or forging operations may be used to achieve the final product shape.
Both of the foregoing procedures permit complex shapes to be achieved, but both are limited to the use of chopped fiber materials as the ceramic matrix reinforcement. For many applications, a composite material wherein the fiber reinforcement is provided in the form of long, parallel-aligned fibers or fiber groups is preferred in order to maximize the strength of the body along a preselected axis in the material.
A procedure for fabricating lightweight reinforced ceramic panels incorporating long reinforcing fibers is disclosed in U.S. Pat. No. 4,822,660. That procedure involves combining fiber-reinforced panels with fiber-reinforced channeled ceramic support elements. The support elements are individually formed by wrapping fiber prepreg on refractory mandrels, and then consolidating the wrappings by hot isostatic pressing to provide dense fiber-reinforced hollow supports.
To provide the completed structure, the hollow supports are combined with the panels by a sealing process. Devitrifiable glass cementing compositions are applied to adjoining surfaces of the supports and panels, and these elements are combined into an assembly which is then fired to bond the elements into a strong, stiff, ceramic composite structure.
While the sealing approach of U.S. Pat. No. 4,822,660 provides a product of relatively high strength and stiffness, the sealed joints in the structure may not provide the same level of structural strength and toughness as the panels and support members themselves. Further, the numerous part fabrication and sealing steps required, and particularly the several high temperature steps involved in consolidation and sealing, disadvantageously increase the cost and complexity of the manufacturing process.
Thus, notwithstanding the significant advances in the art of fabricating complex ceramic matrix composite structures, there remains a need for an efficient and low-cost process for making a composite structure which is light in weight yet strong and highly resistant to bending.
It is therefore a principal object of the invention to provide a new method for the fabrication of a ceramic matrix composite plate or panel of high stiffness and strength.
It is a further object of the invention to provide a ceramic matrix composite article comprising integral stiffening means, and a method for making it, offering high strength and bending resistance at reduced cost.
Other objects and advantages of the invention will become apparent from the following description.